I'm curious, and my search-fu bore me no fruit, as to why higher populations in world generation gravitate towards dictatorships, autocracies, and oligarchies?

I always thought the larger the population, the harder to control, and so would see Balkanization and Representative Democracies be more common. Granted, my understanding could be skewed due to living in a (theoretically) Representative Democracy, but I thought historically as population grows it becomes harder and harder for a single personality to hold power without consent of the masses?

I can definitely see where it's useful when generating Law - the higher the government level (more restrictive / authoritative), the more legal restrictions - but I question tying government type to population in the way that it is.